feat: Link-Template (RFC 9652), link sets (RFC 9264) and api-catalog (RFC 9727)

[Spomky]

Q	A
Branch?	4.4
Tickets	Closes #6924
License	MIT
Doc PR	api-platform/docs#TODO

Three RFCs, three self-contained commits, so they can be split into separate PRs if preferred.

1. Link-Template header (RFC 9652) — closes #6924

Symfony\Component\WebLink\HttpHeaderSerializer skips every link whose href is a URI template:

foreach ($links as $link) {
    if ($link->isTemplated()) {
        continue;
    }
    // ...
}

So a templated web link declared on an operation was silently dropped:

#[Get(uriTemplate: 'books/{id}', links: [new Link('author', '/books/{id}/author')])]

AddLinkHeaderProcessor now serializes those into a Link-Template header:

Link-Template: "/books/{id}/author"; rel="author"; anchor="#{id}"

Full RFC 9652 support: templated anchor, var-base, target attributes as structured field
parameters, and Display Strings for non-ASCII values (title=%"Bj%c3%b6rn J%c3%a4rnsida").

Each header is now only set when its serializer returns something, so a response whose links are
all templated no longer gets an empty Link header.

2. Link set serializer (RFC 9264)

ApiPlatform\State\Util\JsonLinksetSerializer turns a list of PSR-13 links into an
application/linkset+json document, grouped by link context then by relation type.

3. api-catalog well-known URI (RFC 9727)

RFC 9727 mandates the RFC 9264 link set format for the API catalog document, which makes it the
natural consumer of the serializer above.

GET /.well-known/api-catalog

HTTP/1.1 200 OK
Content-Type: application/linkset+json; profile="https://www.rfc-editor.org/info/rfc9727"
Link: </.well-known/api-catalog>; rel="api-catalog"

{"linkset": [
  {"anchor": "https://example.com/.well-known/api-catalog", "item": [{"href": "https://example.com/"}]},
  {"anchor": "https://example.com/",
   "service-desc": [{"href": "https://example.com/docs.jsonopenapi", "type": "application/vnd.openapi+json"}],
   "service-doc":  [{"href": "https://example.com/docs", "type": "text/html"}],
   "service-meta": [{"href": "https://example.com/docs.jsonld", "type": "application/ld+json"}],
   "item": [{"href": "https://example.com/books"}, {"href": "https://example.com/reviews"}]}
]}

Everything is derived from metadata API Platform already has, so there is nothing to configure.
The route is registered with the entrypoint, so enable_entrypoint: false disables it too.

Per section 2, a HEAD request answers with the api-catalog link relation. Per section 3, the
entrypoint advertises the catalog with the same relation, which is what lets clients find it when
the API is mounted under a route prefix and the document is therefore not at the host root. On
Laravel the route is registered outside the API prefix, since RFC 8615 roots well-known URIs at the
host.

Notes

• The two serializers delegate to symfony/web-link 8.2 and later, which gained them in
  [FrameworkBundle][WebLink] Add RFC 9264 link sets and the RFC 9652 Link-Template header symfony/symfony#65428, and fall back to a backport of that implementation otherwise, since
  api-platform supports Symfony 7.4. Every duplicated part is marked with a
  TODO: remove once "symfony/web-link" >= 8.2 is required.
• Two Laravel tests were updated: the entrypoint now carries the api-catalog link relation, which
  is an additive change to the Link header.
• Unrelated, noticed while working on this: HttpOperation::$links only reaches the link provider
  through HydraLinkProcessor, so web links declared on an operation are ignored when Hydra is
  disabled. Left untouched here.